Among the image display panels, there are known a cathode ray tube (CRT), a plasma display panel (PDP), an electroluminescence panel (ELP) etc., utilizing a light-emitting material, and a liquid crystal display (LCD), a dynamic micromirror display (DLP), an electrophoretic display (EPD) etc., utilizing an optical switch element not emitting light.
A high-definition color image display panel is generally constituted of an assembly of plural pixels arranged planarly in a matrix arrangement. Also each pixel is often constituted of subpixels for modulating three lights based on three primary colors of the human vision.
In practice, the aforementioned subpixels are formed by further dividing a pixel in the plane. In such case, since an area of the subpixel for a color display is decreased to a fraction according to the number of division, so that the entire display image becomes darker. In an image display panel of the non-light-emitting type, an illumination with a bright back light or front light is often employed in order to compensate such decrease in brightness. In an image display panel of the non-light-emitting reflective type, not provided with such illuminating means, an anisotropic reflective layer is employed to increase the display brightness in a standard viewing position about in front of the image display panel.
Japanese Patent Application Laid-open No. H11-84403 proposes a two-layer laminated configuration without color filters, formed by a color cell of three primary colors utilizing an areal division but obtained by a liquid crystal containing a dye guest, and a liquid crystal cell containing a black dye guest, thereby increasing a brightness in a white state.
Japanese Patent Application Laid-open No. H10-68967 discloses superposing two layers of a cell which can take one of three primary colors and a colorless transparent state thereby increasing the brightness in a white state and rendering each of the three primary colors brighter than in the case of division into three areas.
On the other hand, plural proposals have been made for a possibility of providing a color image which is bright in principle, without relying on subpixels based on areal division. In a principal proposal, a light modulation layer is formed as a multi-layered configuration, thereby superposing the subpixels in a direction perpendicular to the display plane. For example, a wavelength-selective reflecting layer, for example, based on cholesteric liquid crystals is superposed in plural layers, and a color image is reproduced by an addition color mixing by the subpixels in a direction perpendicular to the image plane. Also in another example, a layer of a material of dichroic property such as guest-host liquid crystals is superposed in plural layers, and a color image is reproduced by a subtraction color mixing in a direction perpendicular to the image plane.
In the handling of image data, such as an image signal of a color television broadcasting, or image recording data of a DVD-video, a DV camera or a digital still camera, a certain data compression is often adopted. As one of the compression technologies, a data reduction in color signals is frequently executed.
FIG. 7 shows a result of measurement of the spatial frequency characteristics of human vision, reported by Sakata and Isono, Television, Vol. 31, p. 29(1976). In the human visionary characteristics, it is known, as illustrated, that the spatial frequency detecting sensitivity for the luminance becomes highest at about 3 cycle/degree, while that for the color hue becomes highest at about 0.5 cycle/degree. Also it is known, among the color hues, that the spatial frequency detecting sensitivity shows a larger decrease at the high frequency side in a yellow-blue complementary color system than in a red-green complementary color system. Since the spatial resolving power of the human for color is lower than the spatial resolving power for luminance, a reduction of the color information is effective in the aforementioned image signal compressing technologies.
Despite that the spatial resolving power for luminance information is relatively high and that for color information is relatively low in the human vision, a prior color image display panel displays color hue information corresponding to a high resolving power. More specifically, the brightness of a pixel is displayed by controlling subpixels which respectively display three primary colors.
Such display not only provides excessive information unnecessary for the human vision, but also is associated with drawbacks of a decrease in a pixel opening ratio and an increase in the process cost, resulting from a finer structure of the subpixels and increased drive wirings therefor. However, in case the size of the subpixel is increased, a pixel formed by an assembly of the subpixels becomes larger, so that the luminance information cannot be displayed with a sufficient resolving power. The present invention is to provide a bright color display panel of a satisfactory display resolving power at a low cost. Thus, the issues to be solved are to secure a brightness of the displayed image, to secure a display resolving power and to reduce a production cost.